Capacitive circuits are used in a multitude of applications, to serve various needs. For example, fringe capacitors are often used in CMOS applications as integrated capacitor with low parasitics to ground.
Radio frequency (RF) circuits exemplify one such type of circuit that uses fringe capacitors. At microwave frequencies, the quality of passives and interconnects is important for the RF circuit performance. Many fringe capacitors use stacked fingers with small finger width and pitch, creating a lateral capacitance between the fingers, and the fingers are placed on top of an N-well.
While fringe capacitors have been useful, their implementation with many applications has been challenging. For example, with RF circuit applications involving microwave frequencies, fringe capacitors often underperform for one or both of single-ended mode and differential mode operation, due to limited quality factor of the fringe capacitor at such microwave frequencies. Due to a variety of factors such as current path length and shrinkage of the lowest metal layers in advanced CMOS processes, these metals exhibit high loss and inductance, which can result in a drop in the quality factor of a fringe capacitor at high (e.g., microwave) frequencies. Accordingly, previous fringe capacitors have exhibited a low quality factor beyond 60 GHz (e.g., less than 10), and the capacitance between connecting pins of the fringe capacitor increases as a function of frequency (e.g., from 95 fF (low frequency) to 115 fF (60 GHz)).
These and other issues continue to present challenges to the implementation of capacitive circuits.